1. Filed of the Invention
The present invention relates to a dual-purpose combustor for ordinary combustion and pulse combustion, especially suited to be installed on a spray dryer. The combustor is used not only as a convenient ordinary noiseless combustion gas generator but also as a pulse combustion gas generator capable of generating hot blasts and high frequency sound waves even at large capacity to keep the high drying efficiency.
2. Description of the Prior Art
Spray dryers (spray drying equipments) are widely employed in food industries, chemical industries and the like, and hot blasts from ordinary burners (LPG burners) are commonly used for their drying heat sources. Ordinary combustors are generally in box shapes (rectangular ducts) disposed in the course of drying air ducts, and have a burner at central portion of the box. Ordinary spray dryers have such an arrangement as "air intake fan.fwdarw.air intake duct.fwdarw.LPG burner.fwdarw.insulated air intake duct.fwdarw.dryer.fwdarw.hot blast chamber", which requires a rather wide installation space and a cost inclusive of insulated air intake duct amounting to 3-5 times more than a LPG burner cost. Further, some materials are dried insufficiently by ordinary combustion gases.
Different from ordinary combustors, pulse combustors generate pulsating high-temperature combustion gases resulting from explosive combustions on scores--several hundred cycle/second. When water-bearing raw materials are sprayed into the combustion gas, the material is subjected to physically impulsive actions (sonic waves and pressure waves) in addition to drying effects by the hot blast. As the result, since a far higher drying rate is available by comparison with the spray drying with ordinary hot blast, attention is given these years to pulse combustors as means for drying raw materials recognized as difficultly dried by conventional hot blast drying equipments.
Pulse combustors are based on the technology of jet engines, and various type of combustors have been proposed for drying of water-bearing materials. As a typical example thereof, the pulse transducer disclosed by JP-B-6-33939 will be explained hereunder by reference to FIG. 6. In the pulse transducer, the combustion chamber 3 having a narrow outlet portion 4 is connected coaxially with an exhaust gas chamber 5 enlarging gradually, and a fuel supply pipe 9, a combustion air supply pipe 10 and ignition means 41 exemplified by a sparking plug are disposed for the combustion chamber 3. When the combustion chamber is filled with air supplied through the combustion air supply pipe 10 and the fuel oil from the fuel supply pipe 9 is sprayed or gaseous fuel like LPG is charged, combustion of the fuel occurs explosively upon ignition and the resulted hot blast proceeds to the exhaust gas chamber 5. The supply of air and fuel is interrupted momentarily due to the temporary high pressure in the combustion chamber 3, but the supply of air and fuel resumes when the combustion chamber 3 turns to reduced pressure conditions caused by exhaustion of the combustion gas to the exhaust gas chamber 5, and phenomenon including the ignition, explosive combustion and blasting of exhaust gas are repeated. The intermittent blasting generates pulsating hot blast and sound waves. When a water-bearing raw material is supplied through the water-bearing raw material supply pipe 15 into the exhaust pipe 5 or supplied at outlet portion of the exhaust pipe 5, the water-bearing raw material is subjected not only to drying effect by the hot blast but also to pulsating physical impacts (sound wave force, pressure, etc.) so as to be dehydrated instantly. In the course of time for thus started pulse combustor, inside walls of the combustion chamber 3 turn to ignited states to result in eliminating ignition of the charged air and fuel with the ignition means 41, and enabling the automatic ignition thereof by contacting with the ignited inside wall to bring about repeated intermittent explosive combustions.
There are valve type combustors and valve-less type ones for pulse combustors, and the former controls the combustion by means of valves disposed at the combustion air intake and at the fuel intake both connected to the combustion chamber. The valve type combustors are able to control the explosive combustion frequency, however, frequencies of up to scores cycle/second are at the best due to the mechanical switching. The valve-less type combustors provide explosive combustion frequency of several hundred cycle/second by small scale combustors of scores thousand kcal/Hr. From the standpoint that more effective drying effects are obtainable when the frequency of explosive combustion becomes higher, valve-less type combustors are considered as superior because of their capabilities for providing higher frequencies and causing no mechanical troubles. However, valve-less type combustors have defects of lowering explosive combustion frequency and decreasing drying efficacy for larger scale combustors, due to the inversely proportional relationship between the explosive combustion frequency and volume of the combustion chamber. Further, a lowered explosive combustion frequency may cause resonance to housings for the installation.
At the price of the superior drying effect, pulse combustors generate far louder noises incomparable to ordinary hot blast drying facilities, and it is required to take sound (noise) prevention measures. From the view point of insulating noises coming from the drying facilities, since insulation of sound (noise) is easier for higher frequency sounds and quite difficult for lower frequency sounds, the maximum drying capacity of around 800,000kcal/Hr are considered as the upper limit for conventional pulse combustors. For a dryer having drying capacity of more than several million kcal/Hr, it is contemplated to dispose a number of small scale pulse combustors at top portion of the drying tower to constitute totally a large capacity drying facility, however, the facility costs too much and the piping system is too complicated. Accordingly, a pulse combustor having a large capacity and a high frequency pulse combustion is desired.
As shown in FIG. 6, when the water-bearing raw material supply pipe 15 is disposed along the central axis of the pulse combustor, the water-bearing raw material supply pipe is heated to 1200.degree. C. or higher, and charring of raw material on inside surface of the water-bearing raw material supply pipe and on the spraying nozzle occurs to develop troubles during a long term operation or a continuous intermittent operation. Even when the water-bearing raw material supply pipe 15 is inserted in a heat insulated protecting tube and forceful blowing of outdoor air into the protecting tube is undertaken, the water-bearing raw material supply pipe cannot be cooled enough. Further, selection of construction materials for the water-bearing raw material supply pipe and heat insulated protecting tube is a problem. As shown in FIG. 7, there is a way of inserting sideward the water-bearing raw material supply pipe 15 and disposing the spraying nozzle at outlet portion of the exhaust gas chamber 5, however, troubles arise during the continuous operation due to the raw material supply pipe and spraying nozzle are heated to form charred particles adhering on surfaces of the pipe and nozzle.
There is another problem that since the allowable combustion capacity range of pulse combustors for keeping the stable pulse combustion is so narrow as around .+-.30% that the range required for ordinary spray dryers of above .+-.50% is unmanageable.
Many existing spray drying equipments are directed usually to general-use machines for drying various kinds of material, and conventional indirect hot air heating methods or continuous direct combustion air heating methods are mainly employed. Though higher drying rates are obtainable by pulse combustion gas drying methods, the high level noise generation makes only a few users agree to convert their dryers for materials manageable with ordinary hot-air drying methods to pulse combustion gas drying methods just merely for improvements in the drying efficiency, and thus pulse combustion methods have not been employed widely.
Other than the noise problem, another difficulty of pulse combustion gas drying methods for those using conventional hot-air spray dryers is that pulse combustors are unsuitable to be installed in combination with wide angle atomizing nozzles as pressurized spraying nozzles and rotary atomizers used commonly in hot blast dryers, since the pulse combustion exhaust gas is blown out with a small diameter and thus only double-fluid atomizing nozzles exhibiting narrow spraying angles are employable.
An improved drying effects obtainable by pulse combustion gas are attractive to those employing conventional hot blast spray dryers because of enabling the usage for drying of water-bearing raw materials which are recognized heretofore as impossible to be dried by spray drying methods. However, the interest by those users is almost lost when they know not only of difficulties in incorporating in existing facilities but also of noise problems and inapplicability to existing liquid atomizing equipments. The problems may be solved by facilities capable of switching the combustion methods from one method to another in accordance with the usage, however, such combustors were not manufactured. The reason is that, although it is possible for conventional pulse combustors to maintain the continuous combustion by means of reducing the air-fuel ratio (amount of air charged/theoretical amount of air necessary for complete combustion of supplied fuel) to below 0.7, a secondary combustion with long flame occurs at the outlet of exhaust pipe, and thus it is impossible for spray dryers to employ pulse combustors installed in conventional manners.